Enhanced Simmer Neutronics Tool for Studying Fast Reactor Distorted Core Configurations

International audienceCore disruptive accidents in fast reactors need to bemonitored carefully since they may lead to possible criticalityconfigurations. However, the worst-case scenariomay have small probability occurrences, but the proof ofit requires multidisciplinary studies. Even with the upgradein computer performance, calculations would requireseveral months on several parallel computers.Accurate calculations with short running times are thusrequired. Updating the neutronics module of SIMMERset up in the 1970s was therefore carried out with thehelp of routines able to handle probability tables forgenerating broad group libraries. The use of such librariestogether with new SIMMER options is now able toproduce reliable results in all sorts of situations whilemaintaining reduced calculation times.Indeed, until now, neutronics calculations from SIMMERgave results quite far from ERANOS ones (differencesin reactivity larger than 1.5 $). The discrepancieswere mainly due to the libraries used. As a consequence,in 2000, an ERANOS module (BISIM) was created togenerate SIMMER nuclear data libraries ( for both crosssections and self-shielding factors) from the ERANOSnuclear data file, thereby reducing the major source ofinconsistencies. Other improvements were added by theJapan Atomic Energy Agency, on the way of calculatingthe transport cross section and on the library group schemeso as to better calculate the k-effective within a reasonabletime frame, but also at the Commissariat à l’EnergieAtomique et aux Energies Alternatives on the b-effectivecalculation. A new option (using the Keepin data) wasimplemented in 2010 in SIMMER.Once all these optimizations were carried out, a comparisonbetween the SIMMER (III for two dimensionsand IV for three dimensions) and ERANOS results wasperformed for a series of disruptive and representativeconfigurations. While the computation time has notchanged significantly, the differences on k-effective betweenERANOS reference route results and SIMMER 16energy-group calculations were drastically reduced by;0.8 $